drm/xe/guc: Add Relay Communication ABI definitions

The communication between Virtual Function (VF) drivers and
Physical Function (PF) drivers is based on the GuC firmware
acting as a proxy (relay) agent.

Add related ABI definitions that we will be using in upcoming
patches with our GuC Relay implementation.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20240104222031.277-7-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>